Improve the layout of the Constants help panel

Switches to using a table.widefat instead of a definition list and loses the awkward show example on hover. Also now includes a line showing what the Constant has been set to if it's been set.
Zebra stripe the table
Merge branch 'refs/heads/master' into constant-help
Minor improvements to the server info tab
Merge pull request #455 from humanmade/constant-help

Improve the layout of the Constants help panel